Automatic load-dependent brake power regulators are used to adjust the brake pressure in the brake cylinders to the respective load condition of a vehicle. This adjustment takes place automatically in that, by means of the brake power regulator, the respective spring deflection which is determined by the axle load or the pressure in the respective air spring bellows which is determined by the axle load, regulates the level of the brake pressure which can be controlled by the brake valve. In the case of commercial vehicles it has been customary for many years to regulate the brake pressure -- in particular on the rear axle, since it is subjected to particularly strong variations with respect to the axle load as a result of variable load conditions.
Since, however, these variable load conditions affect not only the axle load of the rear axle but also more or less the axle load of the front axle, it proved advisable to control also the braking forces acting upon the front-wheel brakes in a certain proportion to the braking forces acting upon the rear-wheel brakes. Also, for many vehicles it is necessary to simultaneously control the front axle in order to meet the legal requirements relative to the brake coordination between the motor vehicle and trailer.
When simultaneously controlling the front axle there is an additional advantage in that the brake linings of the motor-vehicle brakes wear off more evenly in all the brake ranges.
In the meantime, for the joint control of the brake circuit of the front axle, there have become known solutions wherein for the front axle a special intermediate valve is used which is influenced by the rear axle brake pressure. These valves require a considerable additional outlay. In addition, there arose the problem that, in the event of a breakdown of the rear-axle brake circuit, only a very low brake pressure is available for the brake circuit of the front axle. This brake pressure is, however, insufficient to obtain the legally required braking effect on the front-wheel brakes in the event of a brake-circuit failure.
As is known, in order to eliminate this problem, the aforesaid regulating valve is provided with an additional, expensive and by no means trouble-free control mechanism which in the event of a loss of pressure in the rear axle brake circuit switches the regulating valve of the front-axle brake circuit to full pressure flow.
The published German application No. 2,248,923 discloses such an expensive control device for the rear-axle brake circuit, in which case there is provided, for the joint control of the front-axle brake circuit, a special control device which has a constant transmission ratio and which is acted upon by the pressure -- as controlled in the rear-axle brake circuit -- in such a manner that the transmission ratio can be changed in accordance with the controlled pressure in the rear-axle brake circuit. At the same time this control valve is provided with an additional control device, a switch valve, which is acted upon as a function of the pressure in the rear-axle brake circuit, selectively by the controlled pressure in the rear-axle brake circuit or by the uncontrolled pressure of the front-axle brake circuit, in such a manner that, when there is pressure in the rear-axle brake circuit, the controlled pressure thereof is operative and, when the rear-axle brake circuit is without pressure, the pressure in the front-axle brake circuit is operative, i.e. in the event of the rear-wheel brakes breaking down, the front-wheel brakes become fully operative.
U.S. Pat. No. 3,904,253 discloses a load-dependent control device for the rear-axle brake circuit whose pressure generator is in the form of a tandem master cylinder. However, in this control system the front-axle brake circuit is not jointly controlled, but, in the event of a breakdown of the front-axle brake circuit, the brake-power regulating action of the pressure generator in the rear-axle brake circuit is merely fully or partially eliminated so as to obtain a stronger braking effect of the rear-wheel brakes by supplying the full or at least a higher brake pressure to the rear-axle brake circuit.